DEVELOPMENT OF HUMAN PHARYNX 337 



of the vertebrates.' In the search for the primitive type of 

 pharynx one must pass over the higher forms and teleostomes 

 to the elasmobranchs, cyelostomata (larval lamprey), Amphi- 

 oxus and the ascidians, which for a number of reasons must 

 be considered the more primitive of H\'ing chordate forms. There 

 we find, in addition to the lateral branchial region with the 

 gill pouches, well-definied hypobranchial and epibranchial 

 (hyperbranchial) longitudinal zones, each with characteristic 

 ciliated grooves — (or ridges)^ — ^the hypobranchial and epibranchial 

 grooves (Acanthias, larval lamprey) — which again are comparable 

 with the corresponding grooves of Amphioxus and the ascidians. 

 I shall therefore consider the primitive pharynx as consisting 

 morphologically of four zones : the hypobranchial region or zone , 

 the 'floor'; the epi (hyper) branchial region or zone (the roof), 

 and the lateral wall or branchial region proper. As to the num- 

 ber of branchial pockets which the primitive pharynx possessed, 

 there is no way of estimating, nor is the point important in the 

 present connection; there would seem to have been at least eight. 

 While the terms 'hypobranchial' and 'epibranchial' are subject 

 to criticism, particularly the latter, they nevertheless are de- 

 fensible and the former has long been used — as, hypobranchial 

 groove, hypobranchial skeleton, hypobranchial musculature, etc. 

 In considering the fundamental morphological relations of 

 the region we encounter also considerable difficulty in their 

 precise determination. In the first instance, this applies to an 

 exact delimitation of the pharynx caudally, where the pharynx 

 passes more or less insensibly into the esophagus. In the hypo- 

 branchial region the fundamental plan of the vascular relations 

 is quite characteristic. While the heart appears in all cases to 

 occupy a position caudad of the pharyngeal region (infrapharyn- 

 geal), the truncus aorticus is hypobranchial and its bifurcation 

 well forward — as judged by the characteristic relation to the 

 thryeoid gland, at the level of the second arch, although from 

 comparative observations it is clear that the position of the bi- 

 furcation may vary considerably in the adult. 



While the heart seems infrapharyngeal, the pericardial meso- 

 thelium is joined to the mesodermal cords of the branchial 



